Oil progresses to natural gas in deep sedimentary basins. This process, hereafter referred to as “oil-to-gas,” is believed to be the major source of natural gas in the earth (Hunt, Petroleum Geochemistry and Geology, 2nd ed., W. H. Freeman, New York., Chapter 7, 1996). Knowing when and how this process occurs is the key to predicting the distribution of oil and gas with depth. The conventional view is that oil thermally cracks to gas (thermal gas) at temperatures between 150° C. and 200° C., the observed temperature range where most oil-to-gas occurs. Various kinetic models (thermal models) based on this theory have had only marginal success, however, and there are glaring contradictions. Oil, for example, is found in deep reservoirs (>20,000 ft) at temperatures where it should not exist (Paine et al., “Geology of natural gas in South Louisiana,” American Association of Petroleum Geologists, Memoir 9, Volume 1, Natural Gases of North America, Beebe, B. W., Editor, 376–581, 1968; Price, “Thermal stability of hydrocarbons in nature: Limits, evidence, characteristics, and possible controls,” Geochimica et Cosmochimica Acta, 57:3261–3280, 1993), and giant deposits of so-called thermal gas exist in shallow reservoirs that cannot be explained by the thermal model without invoking long-range migration from deeper horizons (Littke et al., “Gas generation and accumulation in the West Siberian basin,” AAPG Bull., 83:1642–1665, 1999).
There is now mounting scientific evidence against the thermal models. From a series of laboratory experiments under realistic conditions (Domine et al., “Towards a new method of geochemical kinetic modeling: implications for the stability of crude oils,” Organic Geochemistry, 28:597–612, 1998; Domine et al., “Up to what temperature is petroleum stable? New insights from 5200 free radical reaction model,” Organic Geochemistry, 33:1487–1499, 2002), evidence now suggests that oil should not crack to gas over geologic time at temperatures between 150° C. and 200° C., the range within which most so-called thermal gas is formed, a conclusion supported by numerous other studies (Mallinson et al., “Detailed chemical kinetics study of the role of pressure in butane pyrolysis,” Industrial & Engineering Chemistry, Research, 31:37–45, 1992; Burnham et al., “Unraveling the kinetics of petroleum destruction by using 1,213C isotopically labeled dopants,” Energy & Fuels, 9:190–191, 1995; Jackson et al., “Temperature and pressure dependence of n-hexadecane cracking,” Organic Geochemistry, 23:941–953, 1995). Moreover, the gas produced in oil cracking is severely depleted in methane and does not resemble natural gas as it is distributed in the earth (Mango, “The origin of light hydrocarbons,” Geochimica et Cosmochimica Acta, 64:1265–1277, 2001).
Catalysis by transition metals is an alternative explanation for oil-to-gas (Mango, “Transition metal catalysis in the generation of petroleum and natural gas,” Geochimica et Cosmochimica Acta. 56:553–555, 1992), and there is experimental evidence supporting it. Crude oils are converted to gas over zero-valent transition metals (ZVTM) (e.g., Ni, Co, and Fe) under moderate laboratory conditions (150–200° C.) and the products are identical to natural gas in molecular and isotopic composition (Mango and Hightower, “The catalytic decomposition of petroleum into natural gas,” Geochimica et Cosmochimica Acta, 61:5347–5350, 1997; Mango and Elrod, “The carbon isotopic composition of catalytic gas: A comparative analysis with natural gas,” Geochimica et Cosmochimica Acta, 63:1097–1106, 1998; Mango, “The origin of light hydrocarbons,” Geochimica et Cosmochimica Acta, 64:1265–1277, 2000).
The above-described experiments are highly relevant to the generation of natural gas in sedimentary basins. Transition metals are common in sedimentary rocks (Boggs, S., Jr., Principles of Sedimentology and Stratigraphy, 2nd ed., Prentice-Hall, Inc., NJ, pages 165 & 195, 1995), and could become catalytically active (i.e., reduced to zero-valencies) given the reducing conditions of petroleum habitats (Mango, “The light hydrocarbons in petroleum: a critical review,” Organic Geochemistry, 26:417–440, 1997; Mango, “The origin of light hydrocarbons,” Geochimica et Cosmochimica Acta, 64:1265–1277, 2000; Medina et al., “Low temperature iron- and nickel-catalyzed reactions leading to coalbed gas formation,” Geochimica et Cosmochimica Acta, 64:643–649, 2000; Seewald, “Organic-inorganic interactions in petroleum-producing sedimentary basins,” Nature, 426:327–333, 2003). All requisites are in place: transition metal oxides in sufficient amounts to promote the reaction and enough hydrogen to activate them to zero-valencies and drive the reaction at subsurface temperatures (Mango, “The origin of light hydrocarbons,” Geochimica et Cosmochimica Acta, 64:1265–1277, 2000).
Catalysis may be the source of the huge gas deposits in the Gulf Coast geosyncline of south Louisiana (Paine et al., “Geology of natural gas in South Louisiana,” American Association of Petroleum Geologists, Memoir 9, Volume 1, Natural Gases of North America, Beebe, B. W., Editor, 376–581, 1968). Oil is generally found at depths above 10,000 feet and gas is generally found below such depths, consistent with the thermal model. However, gas probabilities are also a strong function of reservoir composition: low in pure sandstone and high in sandstones interbedded with outer-neritic shales that are often enriched in transition metals (Mann and Stein, “Organic facies variations, source rock potential, and sea level changes in Cretaceous black shales of the Quebrada Ocal, Upper Magdalena Valley, Colombia,” American Association of Petroleum Geologests Bulletin, 81:556–576, 1997; Cruickshank and Rowland, “Mineral deposits at the shelfbreak,” SEPM Special Publication No. 33, 429–436, 1983).
Given high enough temperatures and hydrogen partial pressures at depth, transition metals in outer-neritic shales could attain zero-valencies. Thus activated, in-reservoir catalytic oil-to-gas would commence. In this instance, the important factor for predicting oil or gas in reservoir rocks is the presence of ZVTM in sufficient concentrations to promote catalytic oil-to-gas. A rock assay specific to ZVTM in outcrop rocks, cuttings, or core samples would thus be a powerful exploration tool for reservoirs that either preserve oil (no ZVTM) or convert it to gas (with ZVTM).
Other than commonly assigned co-pending U.S. patent application Ser. No. 10/830,266, Applicant is unaware of any practical tests for trace amounts (i.e., ppb or less) of ZVTM in sedimentary rocks. Most rock methods use spectroscopic techniques, such as atomic absorption (AA) spectroscopy or inductively-coupled plasma atomic emission spectroscopy (ICP-AES), that do not differentiate between oxidation states. Nickel valency speciation has been achieved by X-ray absorption fine-structure spectroscopy using the National Synchrotron Light Source at Brookhaven National Laboratory (NY) and with anodic stripping voltammetry (Galbreath et al., “Chemical speciation of Nickel in residual oil ash,” Energy & Fuels, 12:818–822, 1998), but the complexities of these methods preclude their use in routine rock analysis.
In addition to the above, a convenient assay for the direct determination of intrinsic paleocatalytic activity within sedimentary rock, for the purpose of making predictions in oil and gas exploration, would also be highly desirable. Applicant is unaware of any assays that measure the intrinsic catalytic activity of rocks to convert oil to gas under subsurface conditions.